1. Field of the Invention
This invention relates to a method of separating large-sized film-like plastics.
2. Description of the Prior Art
The quantities of film-like plastics in industrial and household wastes have recently been increasing, but no effective method for treating these waste plastics has been established. Only incineration and the melt-remolding method are in current use. Incineration, however, is not a desirable method because polyvinyl chloride contained in the plastics can cause secondary pollution due to evolution of harmful gases or harmful metal oxides.
Methods have been described for separating and recovering polyvinyl chloride from mixed plastics using flotation as disclosed in copending U.S. Patent Applications Ser. No. 443,937, filed Feb. 19, 1974, Ser. No. 443,936, filed Feb. 19, 1974 and Ser. No. 443,574, filed Feb. 19, 1974. The methods described in these applications are directed to the treatment of mixtures of granular plastics of a relatively small size using flotation. Since flotation is a process whereby particles are raised to the water surface due to the buoyancy of fine gas bubbles adhering to the particles, the method is directed to and was developed for particles of a relatively small particle sizes. With ores, fine powders with a particle size of less than about 0.3 mm can usually be treated using this method, and on very rare occasions, particles of less than several millimeters can be floated. However, it is impossible to raise larger particles using the buoyancy of gas bubbles.
The inventors made a number of studies on the flotation characteristics of plastics, and found that the manner of adhesion of the gas bubbles differs depending on the surface characteristics of film-like plastics in the water, and where adhesion of a large number of gas bubbles to the surfaces can be achieved, even large-sized film-like plastics can be easily raised and floated on the water surface. Films such as polyvinyl chloride or polystyrene films having a specific gravity larger than 1 sink to the bottom in a tank containing water. When a number of small gas bubbles are introduced into the tank, these small gas bubbles begin to adhere to the surface of the films and the number of gas bubbles so adhered gradually increases. Eventually, the total buoyancy of the gas bubbles surpasses the gravity of the films, and the film rise in the water and float on the surface of the water in such a manner that their maximum surface area is substantially aligned with the surface of the liquid medium. Films having a specific gravity of less than 1, such as polypropylene or polyethylene films, gradually rise in the water and reach the water surface even when no specific measure is taken. However, when gas bubbles adhere to such films, the rate at which the films rise greatly increases, and such films rise to the water surface at a speed about 5 to 10 times as fast as that attained in the absence of the buoyancy effect generated by the gas bubbles thus making it possible to drastically shorten the time required for separation. If the separating time is so shortened, large quantities of material can be treated in a small-scale apparatus.